A radiometer is a detector capable of sensing and measuring the radiant power or intensity of electromagnetic radiation incident thereon. In the infrared portion of the electromagnetic spectrum, radiometers usually act as thermal cameras. In this case, a thermal imaging radiometer is adapted to record incoming infrared radiation emitted by the objects of a target scene encompassed by the field of view of the instrument. The recorded radiation is converted into electrical signals, which are processed in order to provide a thermal infrared image displaying the spatial temperature distribution of the scene under observation. Thermal infrared imaging radiometers typically include optics that collect radiation emitted from a target scene and focus that radiation onto a linear or two-dimensional focal plane array (FPA) of elemental temperature-sensing detectors, such as uncooled infrared microbolometers.
Microbolometers are thermal detectors capable of operating at room temperature. They are relatively inexpensive to produce and since they do not require cryogenic cooling, they can be integrated within compact and robust devices that are less expensive than cooled detectors. They can be used in a wide variety of applications, including night vision, observation of the Earth from space, pollution and fire detection, spectroscopy, and border control.
Referring to FIGS. 1A and 1B (PRIOR ART), there are shown schematic top and side cross-sectional views, respectively, of a typical thermal imaging radiometer 20. The thermal imaging radiometer 20 includes an array of imaging microbolometers 22 disposed on a substrate 34 and enclosed inside a vacuum package 36. The vacuum package 36 includes a window 50 allowing incoming radiation 54 to reach the imaging microbolometers 22.
Each imaging microbolometer 22 of the array typically includes a sensor element 40 and supporting legs 42 that thermally isolate the sensing element 40 from the substrate 34, typically by suspending it thereabove. As is known in the art, the electrical resistance of the sensor element 40 changes in response to a variation of temperature thereof, which in turn varies as a function of the amount of radiation 54 absorbed thereby. In other words, radiation 54 impinging on the sensor element 40 of each imaging microbolometer 22 of the array increases the temperature of the sensor element 40 and causes a corresponding variation of its electrical resistance.
The substrate 34 on which lies the array of imaging microbolometers 22 may further include an imaging electrical readout circuit 30 adapted to convert the resistance change of each imaging microbolometer 22 into a corresponding electrical output (e.g., a voltage or a current output). This electrical output may be subsequently processed (e.g., amplified, multiplexed, filtered, digitized, and the like) into temperature data by a processing unit 38 connected to the imaging readout electrical circuit 30. The temperature data can then be used to generate a pixelated thermal image of a target scene, wherein each pixel of the thermal image is associated with a corresponding imaging microbolometer 22 of the array. This thermal image may be displayed on a screen or stored for later use or processing.
Generally, in order to yield reliable temperature measurements, each imaging microbolometer must be properly calibrated. This may be accomplished, for example, by exposing the imaging microbolometer array to radiation from a blackbody calibration source, and by sensing the electrical output of each imaging microbolometer of the array in response thereto. The procedure may be repeated with a set of blackbody calibration sources maintained at various temperatures in order to cover a desired temperature range. This initial calibration is most often performed in a factory setting during the manufacturing process of the radiometers or prior to a first use thereof, so as to provide the proper conditions required for accurate calibration. Once calibrated, each microbolometer of the array is provided with an individual response function that relates the electrical output response of the microbolometer associated therewith to the temperature of the specific part of the scene imaged thereon. This response function is stored and retrieved during operation of the radiometer in order to convert the electrical output generated by each imaging microbolometer into temperature data that may be used to form a thermal image of a target.
However, once a microbolometer is calibrated, the manufacturer cannot guarantee, and often underestimates, the time during which the calibration will remain valid. For this reason, radiometers need to go through frequent periodic recalibration runs so as to ensure optimal performance and to correct calibration drifts which, over time, inevitably start degrading the accuracy and sensitivity of measurements. Calibration drifts in the response of radiometers can be neither detected nor easily corrected in the field since they require exposition to blackbody calibration sources under controlled environmental conditions. Therefore, these frequent periodic recalibration runs often need to be performed at the premises of the manufacturer, thus increasing maintenance costs while making the radiometers unavailable for their intended use, sometimes for extended periods of time. Moreover, spare radiometer units may be made available to allow for uninterrupted field operations, which may amount to a substantial fraction of the inventory of radiometers used simultaneously in the field, thus further increasing the costs associated with the recalibration runs. The overall costs of recalibrating radiometers may thus be as high as those incurred during the initial calibration thereof.
In view of the above considerations, there exists a need in the art for a method for detecting a loss of calibration of microbolometer thermal imaging radiometers that could be performed in the field without having to use an external calibration setup. There also exists a need in the art for a method capable of correcting a loss of calibration of such radiometers without having to go through a full recalibration procedure involving blackbody calibration sources in a controlled factory setting.